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VFX Match-move Question
  • soapeysoapey

    Hi GH2ers,

    I have a very technical question directed toward those of you who use your GH2 to shoot video for CG and Match-Moved VFX.

    When I need to camera track and/or camera project in Nuke or Maya, it is best to put it in the 35mm equivalent focal length of the lens. This is pretty simple with the GH2 since the focal length is just twice the focal length of the lens. EXCEPT, what happens when you shoot in Ex-Tele-Convert mode?

    On the one hand the amount of lens distortion seems like it would be the same because the camera's just cutting into the image exposed on the sensor. On the other, It just doesn't seem right that that I should be putting in say, 180mm when the recorded image does not match a 180mm image.

    Anybody have some insight into this?

    Thanks for any help.

  • 3 Replies sorted by
  • StrayStray

    Film back size and focal length are used to calculate your lenses field of view when tracking, getting accurate lens distortion information is a lot more important really. That and any real world measurements you may have that could be used to refine your camera solve. Also watch for rolling shutter problems, that can obviously really knacker any track.

    Even if you do know the lenses focal length its rarely a good idea to set this to a fixed value while tracking. If you are using a 35mm lens then I'd say set it to what it really is (dont multiply it) as the ballpark starting figure, but let the software treat it as a variable lens. Let it auto-detect, check the quality of the solve and the figures it comes up with. Make sure you have a scene with enough parallax http://en.wikipedia.org/wiki/Parallax too for your first attempt at a solve with a specific lens, just to get a feel for it and to give the solver a decent chance. ETC mode though, I've no idea, but you could set it based on film back, or focal length (you're still just calculating the field of view).

    You can also manually key some 2D transforms in nuke later to account for small drifts (or set up a 2D tracker to help). Also take an undistorted plate into Maya to work against, apply the lens distortion to your CG afterwards when you comp.

    In short, get good lens distortion info by shooting a grid with the lens at the right focal length, let the software auto detect and see what you get. Then tweak field of view related values if that helps the solve. Make constraints on tracked points that represent a ground plane (or any consistent plane) if you can.

  • soapeysoapey

    Thanks Stray. Just out of curiousity (and for testing purposes to see if my tracks come out more accurate - PFTrack supposedly works better when it knows the actual filmback size) does anybody know the size of the area of the sensor that is being exposed when using ETC mode?

  • StrayStray

    @soapey ETC mode 7.31*4.12mm

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